The invention relates in general to means for correcting dead time losses in a random event counter circuit having a dead time characteristic. More particularly, the present invention relates to a Geiger-Mueller tube-based analog rate meter circuit of the charge pump type having correction means for compensation of Geiger-Mueller tube dead time losses experienced primarily at high count rates.
It is recognized that currently available Geiger-Mueller tubes are not 100% efficient, i.e., a Geiger-Mueller tube, even if assumed to be without a dead time characteristic, cannot provide an output pulse for each and every detectable atomic particle impinging on it. However, for purposes of the following discussion, it will be assumed that a Geiger-Mueller tube or other random event detecting transducer is 100% efficient but for any dead time losses. Thus, the following teachings with regard to the present invention apply regardless of transducer efficiency where the random event detecting transducer and/or its associated circuitry exhibits a dead time characteristic as discussed below.
Ideally, a Geiger-Mueller tube, or other transducer for detecting random events, should provide an output signal, such as a pulse, for each atomic particle emission (ionizing radiation) or other detectable random event to which it is exposed. In such an ideal case, the number of output signals, e.g., pulses, from the transducer would correspond in one-to-one relationship (i.e., a linear, directly proportional manner) to the number of detected atomic particles or other detectable random events impinging on the transducer.
It is recognized by those skilled in the art that such an idealized case cannot occur in practice, since the recovery time of the transducer (e.g., a Geiger-Mueller tube) has a limiting effect on its ability to respond to each and every detectable random event that it is exposed to regardless of the brevity of the time period separating successive random events. In the case of a Geiger-Mueller tube, for example, the recovery time (or non-responsive "dead time") of the tube and its associated detector circuitry can be greater than the time period during which two or more detectable (i.e., ionizing) atomic particles (e.g., gamma, alpha, or beta particles) impinge on the tube. In such a case, the tube will generate an erroneous output signal (e.g., a single pulse) indicative of a single particle detection when in fact the tube has been exposed to two or more detectable atomic particles. It is further recognized by those skilled in the art that the occurrence of such erroneous output signals (dead time losses) increases as the frequency of the randomly emitted ionizing atomic particles impinging on the Geiger-Mueller tube increases. Thus, the Geiger-Mueller tube and associated detector circuitry responds in a non-linear manner to the increasing frequency of detectable atomic particle emissions to which it is exposed. Therefore, the count rate measured by the detector circuitry will not accurately indicate the actual count rate of the random event unless some form of correction is provided to compensate for dead time losses. An example of a Geiger-Mueller-tube-based radiation detector not incorporating means for dead time compensation is a Model E-520 radiation detector manufactured and sold by Eberline Instrument Corp., a division of Thermo Electron Corp., Sante Fe, N. Mex. It is an object of the present invention to improve over such prior art detectors.
A more detailed explanation and analysis of dead time loss problems inherent in radiation detectors may be found on pages 95-102 of the text entitled "Radiation Detection and Measurement," authored by Glenn F. Knoll and published by John Wiley & Sons. Such text portion is incorporated by reference herein in its entirety.
U.S. Pat. No. 4,311,909 to Utting et al proposes a technique for dead time loss compensating in a Geiger-Mueller tube-based analog rate meter circuit of the charge pump type. In accordance with Utting et al, a dead time compensating analog switch, opening and closing in response to each event count signal from associated Geiger-Mueller tube-band circuitry, controls the discharge of an integrating capacitor associated with an operational amplifier whose output drives an analog meter intended to accurately indicate the actual rate of events occurring, i.e., the number of detectable atomic particles per unit of time inpinging on the associated Geiger-Mueller tube. If the dead time loss correction circuit requirements set forth by Utting et al are met, Utting et al conclude that their analog rate meter will provide an accurate reading corresponding to the true event count rate. It is implied by utting et al that their rate meter will respond accurately in a linear manner to increases and decreases in the actual event count rate, since such a result is the expected goal of dead time loss compensation. However, it has been found that a Model 400 radiation detector, manufactured and sold by Victoreen Incorporated, a division of Sheller-Globe Inc. of Toledo, Ohio, and purported to be made in accordance with the teachings of Utting et al U.S. Pat. No. 4,311,909, responds in a non-linear manner to actual event count rate increases and decreases, thus necessitating the need for a non-linear meter scale on the face of the meter that is believed to be calibrated empirically, i.e., by experiment, in an attempt to provide accurate results. Some skilled in the art would find the need to empirically calibrate the Utting et al non-linear meter scale to be unacceptable. Further, even if the need to empirically calibrate the Utting et al meter scale is acceptable, the resultant non-linear meter scale may lead to inadvertent misreading by an end user accustomed to reading linear scale readouts.
It is a general goal of the present invention to provide a technique for dead time loss compensation wherein the output of a random event counter incorporating such technique of dead time loss compensation responds in a linear or near linear manner to increases and decreases in the frequency of actual random events being counted as a function of time. It is a specific goal of the present invention to provide a dead time loss compensation circuit for a Geiger-Mueller tube-based analog rate meter circuit of the charge pump type wherein the output of the rate meter circuit responds in a linear or near linear manner to increases and decreases in the actual number of atomic particles per unit time to which the Geiger-Mueller tube is exposed.